香港科技大学：Advanced Topics in Next Generation Wireless Networks

Advanced topics in Next- Generation wireless Networks MAC and 802.11 Qian Zhang Department of Computer science HKUST

Advanced Topics in Next￾Generation Wireless Networks Qian Zhang Department of Computer Science HKUST MAC and 802.11

Multiple Access Links and Protocols Two types of links": point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast(shared wire or medium) old-fashioned ethernet upstream HFC 802.11 wireless LAN humans at a shared wire(e.g shared rI shared RF cocktail party cabled Ethernet) (e.g., 802.11 WiFi) (satellite (shared air, acoustical)

Multiple Access Links and Protocols Two types of “links”: • point-to-point – PPP for dial-up access – point-to-point link between Ethernet switch and host • broadcast (shared wire or medium) – old-fashioned Ethernet – upstream HFC – 802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical)

Multiple access protocols Single shared broadcast channel Two or more simultaneous transmissions by nodes interference Collision if node receives two or more signals at the same time Multiple access protocol Distributed algorithm that determines how nodes share channel i.e. determine when node can transmit Communication about channel sharing must use channel itself no out-of-band channel for coordination

Multiple Access protocols • Single shared broadcast channel • Two or more simultaneous transmissions by nodes: interference – Collision if node receives two or more signals at the same time Multiple access protocol • Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit • Communication about channel sharing must use channel itself! – no out-of-band channel for coordination

Ideal Multiple access protocol Broadcast channel of rater bps 1. When one node wants to transmit it can send at rate r 2. When m nodes want to transmit each can send at average rate r/m 3. fully decentralized: no special node to coordinate transmissions no synchronization of clocks slots 4. Simple

Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: – no special node to coordinate transmissions – no synchronization of clocks, slots 4. Simple

MAC Protocols: a Taxonomy Three broad classes Channel Partitioning divide channel into smaller pieces"(time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided allow collisions recover" from collisions Taking turns nodes take turns but nodes with more to send can take longer turns

MAC Protocols: a Taxonomy Three broad classes: • Channel Partitioning – divide channel into smaller “pieces” (time slots, frequency, code) – allocate piece to node for exclusive use • Random Access – channel not divided, allow collisions – “recover” from collisions • “Taking turns” – nodes take turns, but nodes with more to send can take longer turns

Channel partitioning mac protocols TDMA TDMA: time division multiple access access to channel in rounds each station gets fixed length slot (length pkt trans time)in each round unused slots go idle example: 6-station LAN, 1, 3, 4 have pkt, slots 25.6 idle 6-slot rame

Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access • access to channel in "rounds" • each station gets fixed length slot (length = pkt trans time) in each round • unused slots go idle • example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 1 3 4 1 3 4 6-slot frame

Channel partitioning mac protocols FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1, 3, 4 have pkt frequency bands 25.6 idle time ∩00 FDM cable

Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access • channel spectrum divided into frequency bands • each station assigned fixed frequency band • unused transmission time in frequency bands go idle • example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands FDM cable

Random access protocols When node has packet to send transmit at full channel data rate r no a priori coordination among nodes Two or more transmitting nodes + collision Random access mac protocol specifies how to detect collisions how to recover from collisions(e. g. via delayed retransmissions) Examples of random access mac protocols slotted AlOHa ALOHA CSMA, CSMA/CD, CSMA/CA

Random Access Protocols • When node has packet to send – transmit at full channel data rate R. – no a priori coordination among nodes • Two or more transmitting nodes ➜ “collision”, • Random access MAC protocol specifies: – how to detect collisions – how to recover from collisions (e.g., via delayed retransmissions) • Examples of random access MAC protocols: – slotted ALOHA – ALOHA – CSMA, CSMA/CD, CSMA/CA

Slotted aloha Assumptions: Operation: o all frames same size When node obtains fresh ● time divided into frame, transmits in next equal size slots(time slot to transmit 1 frame) - if no co∥ IS/on:node nodes start to can send new frame transmit only slot in next slot beginning -ifCo∥ lision;node nodes are retransmits frame in synchronized each subsequent slot ●f2 or more nodes with prob. p until transmit in slot,a‖l success nodes detect collisⅰon

Slotted ALOHA Assumptions: • all frames same size • time divided into equal size slots (time to transmit 1 frame) • nodes start to transmit only slot beginning • nodes are synchronized • if 2 or more nodes transmit in slot, all nodes detect collision Operation: • when node obtains fresh frame, transmits in next slot – if no collision: node can send new frame in next slot – if collision: node retransmits frame in each subsequent slot with prob. p until success

Slotted aloha nods[1[[[1 node 2 2 node 3 alot C E C S E C E S Pros Cons single active node can collisions wasting slots continuously transmit idle slots at full rate of channel nodes may be able to highly decentralized detect collision in less than time to transmit only slots in nodes need to be in sync packet clock synchronization sImple

Slotted ALOHA Pros • single active node can continuously transmit at full rate of channel • highly decentralized: only slots in nodes need to be in sync • simple Cons • collisions, wasting slots • idle slots • nodes may be able to detect collision in less than time to transmit packet • clock synchronization